The reaction chamber of a fluidized bed reactor typically comprises an inner portion having a rectangular horizontal cross section, defined by four side walls, a bottom portion and a roof portion, wherein solid material and bed material containing, for example, fuel, are fluidized by means of fluidizing gas introduced through the bottom, the fluidizing gas usually being oxygenous primary gas required for exothermic chemical reactions taking place in the reaction chamber. The inner portion of the reaction chamber is usually called a furnace and the reactor a fluidized bed boiler, when a combustion process is performed in the reactor. The side walls of the furnace typically also have connections for the introduction of, at least, fuel and secondary air.
The side walls of the furnace are usually manufactured of panels formed of tubes and fins therebetween, whereby energy being released in the chemical reactions of the fuel is used for evaporating water flowing in the tubes. Often, there are also superheating surfaces arranged in the fluidized bed reactor to further increase the energy content of the steam.
When aiming to manufacture a high capacity boiler, for example, one on the order of several hundreds of megawatts, a large reaction volume and a lot of evaporating and superheating surfaces are required. Such a high capacity fluidized bed boiler is disclosed in U.S. Pat. No. 6,470,833 B1. The basal area of the boiler is directly proportional to the capacity of the boiler, on the basis of the required volume and velocity of the fluidization air. As it is at least structurally disadvantageous to have a very long and narrow bottom of the furnace, the height of the boiler and the width of the boiler bottom also have to be increased in order to have enough evaporation surfaces on the side walls. The increase of the height may significantly lead to constructional difficulties, and the increase of the width can make it difficult to arrange homogeneous feed of the fuel and secondary air. It can be difficult to form sufficiently strong and rigid side walls extending from the bottom to the roof of a high furnace, as the side walls are considerably thin relative to their height.
It is especially challenging to realize a high-efficiency once-through fluidized bed boiler. The increase of the cross-sectional area of the furnace makes it challenging to maintain uniform behavior of the fluidized bed. This means, in practice, that the heat surfaces of the furnace tend to be affected by a varying fluidized bed, depending, for example, on the structures of the grid and the lower portion of the furnace and the control of the process. It is important for the reliable operation of the once-through fluidized bed boiler that the evaporation of water in the tubes of the evaporator surfaces is homogeneous enough in the different portions of the furnace walls. In large fluidized bed boilers, especially, in the once-through fluidized bed boilers, the uniformity of the fluidized bed has an even bigger meaning. Especially, the inner corners of a large boiler are areas, in which the effect of the fluidized bed on the evaporation is easily different from that in the other areas.